1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a recording medium using an image bearing member such as an intermediate transfer member, and a method for controlling recording medium conveyance in an image forming apparatus.
2. Description of the Related Art
In recent years, image forming apparatuses such as laser printers and copying machines are required to form images at high speed to achieve high throughput, to form high-quality images, to have multiple functions, and to be able to form images on various types of recording media (hereinafter referred to as sheets).
For example, in color laser printers, a method is employed in which an intermediate transfer member capable of bearing a plurality of developer images is used. This method can increase the number of images formed per unit time, and is suitable for improving image quality when forming color images. In this method, a developer image is formed using developer (for example, toner) on a photosensitive drum serving as an image bearing member, the developer image is primarily transferred to an intermediate transfer member, and then the developer image is secondarily transferred from the intermediate transfer member to a sheet.
In the case of such a configuration, the intermediate transfer member and a secondary transfer member for transferring the developer image from the intermediate transfer member to the sheet are pressed against each other at a predetermined pressure and form a pressure contact portion (hereinafter referred to as nip). When the sheet enters the nip, load variation can occur in the intermediate transfer member.
Particularly when a sheet of heavy paper enters the nip at high velocity, the load variation is significant. This load variation can cause deformation of drive transmission members, such as gears, and significant velocity variation in the intermediate transfer member. If significant velocity variation occurs in the intermediate transfer member when the developer image on the photosensitive drum is transferred (primarily transferred) to the intermediate transfer member, density variation occurs in the developer image and results in a defective image. To prevent such a defect image from being formed, the velocity variation of the intermediate transfer member needs to be minimized.
To minimize the velocity variation due to the load variation, the material of gears can be changed to a high-rigidity one that is difficult to deform. However, in general, if the rigidity of gears is increased, defect image formation from other causes, such as banding, is likely to occur. In general, a material having a rigidity without negative effect such as banding cannot sufficiently curb the velocity variation. It is difficult to select the optimum material without negative effect.
Japanese Patent Laid-Open No. 11-52743 discloses a configuration in which a secondary transfer member that transfers a developer image from an intermediate transfer member to a sheet, is rockably supported, and the secondary transfer member is rocked when the sheet enters the nip between the intermediate transfer member and the secondary transfer member. Due to this configuration, the load variation of the intermediate transfer member can be curbed, and the velocity variation can be reduced. Japanese Patent Laid-Open No. 2007-147758 discloses an art in which, when the leading edge of a sheet enters the nip, the sheet is accelerated at a predetermined rate so as to curb the velocity variation of the intermediate transfer member.
However, in the case of the art disclosed in Japanese Patent Laid-Open No. 11-52743, since the secondary transfer member rocks when the sheet enters the nip, the efficiency of transferring the developer image to the sheet decreases, and defect image formation can occur. In addition, since mechanism elements for rocking the secondary transfer member are added, increase in cost is inevitable. Alternatively, the load variation of the intermediate transfer member when the sheet enters the nip can be reduced by reducing the pressure in the nip between the intermediate transfer member and the secondary transfer member. However, also in this case, defective image formation due to decrease in transfer efficiency becomes a problem.
In the case of the art disclosed in Japanese Patent Laid-Open No. 2007-147758, since the sheet enters the nip in the middle of changing the speed of the motor, the load variation of the intermediate transfer member due to the entry of the sheet destabilizes the rotation of the motor, and a step-out of the motor can occur.